Chapter 2: Biological Classification Application Questions | biology Class 11 CBSE

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Chapter 2: Biological Classification Application Questions | biology Class 11 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A microbiologist discovers an organism living in a highly acidic hot spring at 80°C. The organism has a unique cell wall not made of peptidoglycan. With reference to the Five Kingdom Classification, identify the group this organism belongs to and justify your answer.

The organism belongs to Archaebacteria within Kingdom Monera, specifically the thermoacidophiles — Archaebacteria that thrive in hot, acidic environments.
Its unique cell wall composition (different from peptidoglycan-based eubacterial walls) is the defining molecular feature of Archaebacteria, responsible for their resistance to extremes of temperature and pH.
Being prokaryotic (no nuclear membrane), it belongs to Kingdom Monera, but the extreme habitat and atypical cell wall differentiate it from eubacteria.
This example illustrates how structural adaptations (atypical cell wall and membrane chemistry) allow Archaebacteria to colonise habitats inaccessible to all other life forms.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

During a water quality assessment, scientists find no lichen growth in an urban industrial area but abundant lichen growth in a rural forest nearby. With reference to the nature of lichens, explain what this observation indicates and why lichens are useful for such assessments.

The absence of lichens in the industrial area and their abundance in the clean rural forest indicates that the industrial area has significantly polluted air, particularly high levels of sulphur dioxide and other atmospheric pollutants.
Lichens are extremely sensitive to air pollution because both partners — the photosynthetic phycobiont and the structural mycobiont — are disrupted by pollutants, which inhibit photosynthesis and damage the mutualistic relationship.
Lichens do not grow in polluted environments and therefore serve as reliable biological indicators (bioindicators) of air quality — their presence or absence provides a natural, cost-effective measure of environmental health.
This use of lichens as pollution indicators is an example of applied ecology, where the biological characteristics of organisms are utilised for environmental monitoring without expensive instrumentation.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 3

Applied Concept

A patient presents with neurological symptoms and is diagnosed with a fatal degenerative brain disease. Laboratory tests confirm the infectious agent contains no nucleic acid. With reference to the chapter, identify the agent and explain why it is challenging to classify and treat.

The infectious agent is a prion — an abnormally folded protein that causes fatal neurological diseases. Based on the symptoms and disease context, the patient likely has Creutzfeldt–Jacob disease (CJD), the human analogue of bovine spongiform encephalopathy (BSE).
Prions are challenging to classify because they contain no nucleic acid (neither DNA nor RNA), defying the conventional understanding that infectious agents must carry genetic material to replicate and propagate.
Treatment is extremely difficult because prions are abnormal conformations of a host protein; targeting them risks damaging normal host proteins, and there are currently no effective therapies to halt or reverse prion-induced neurodegeneration.
The existence of prions challenges the Five Kingdom Classification, as they are neither cells nor viruses, and their mode of propagation (causing normal proteins to misfold) is a unique mechanism of infectivity not seen in any other biological category.

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Application Question

Easy difficulty • Concept in a practical situation

Easy

Question 4

Applied Concept

Bread left at room temperature in a warm, humid kitchen develops a black fuzzy growth within two days. With reference to Kingdom Fungi, explain the type of organism causing this, its mode of nutrition, and the conditions that favour its growth.

The black fuzzy growth is caused by Rhizopus stolonifer (bread mould), a member of Phycomycetes within Kingdom Fungi, characterised by aseptate coenocytic mycelium and sporangia containing spores.
Rhizopus is a saprophyte — it absorbs soluble organic matter from the dead substrate (bread) through its hyphae, secreting digestive enzymes that break down the complex molecules of bread into absorbable simpler forms.
The warm and humid kitchen environment provides ideal conditions for fungal growth, as fungi prefer warm and moist places. The moist bread surface provides both organic substrate and water for fungal colonisation.
The rapid spread reflects asexual reproduction by sporangiospores released from sporangia at the tips of sporangiophores — these spores are airborne and readily settle on exposed food surfaces, making fungal food spoilage a common challenge.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 5

Applied Concept

An aquatic ecologist observes that a small lake has suddenly turned red and fish mortality has increased sharply. With reference to the organisms discussed in this chapter, identify the cause and explain the biological mechanism behind this event.

The reddening of the lake and fish mortality is caused by a red tide event, triggered by the explosive multiplication of red-pigmented dinoflagellates such as Gonyaulax — photosynthetic, marine/freshwater protists belonging to Kingdom Protista.
When environmental conditions favour their growth (excess nutrients, warm water), dinoflagellates multiply so rapidly that they form dense blooms, imparting a red colour to the water due to their dominant red pigments.
The massive bloom releases potent toxins into the water. These toxins accumulate in the gills and tissues of fish and other marine organisms, disrupting their physiology and causing widespread mortality.
This event illustrates the ecological danger of eutrophication and harmful algal blooms: an organism that is ordinarily a component of the aquatic food web can become destructive when its population grows unchecked, highlighting the importance of monitoring water quality.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

Farmers in a village noticed that the soil in their paddy fields became naturally fertile over time without any chemical fertiliser addition, particularly in areas where blue-green algae were growing. With reference to this chapter, explain the scientific basis for this observation.

The fertility of paddy fields is enhanced by cyanobacteria (blue-green algae), eubacteria belonging to Kingdom Monera, that fix atmospheric nitrogen in specialised cells called heterocysts.
Species such as Nostoc and Anabaena, which are filamentous cyanobacteria common in paddy fields, convert atmospheric nitrogen (N₂) into ammonia and other nitrogenous compounds through biological nitrogen fixation.
These nitrogen compounds are released into the soil upon decomposition of the cyanobacteria, enriching the soil with bioavailable nitrogen — an essential macronutrient for plant growth — without any external chemical input.
This makes cyanobacteria invaluable as natural biofertilisers, providing an eco-friendly and cost-effective alternative to synthetic nitrogen fertilisers, particularly for rice cultivation where waterlogged conditions favour cyanobacterial growth.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 7

Applied Concept

A biology student notices that Euglena placed in a bright environment remains green and photosynthetic, but when kept in a dark box, it begins consuming other microorganisms. With reference to this chapter, explain which kingdom Euglena belongs to and why this dual behaviour makes its classification debatable.

Euglena belongs to Kingdom Protista, specifically the group Euglenoids. It is classified here as a single-celled eukaryote, though its nutritional behaviour spans both plant-like and animal-like modes.
In bright light, Euglena is photosynthetic (autotrophic) using pigments identical to those in higher plants. In darkness, it becomes heterotrophic, predating on smaller organisms — a behaviour typically associated with animals.
This dual nutrition mode, combined with the absence of a cell wall (replaced by a protein-rich pellicle) and the presence of two flagella, means Euglena does not fit neatly into Plantae or Animalia — making it a classic example of the difficulty in classifying unicellular organisms.
Euglena exemplifies why Kingdom Protista was created: to accommodate organisms that share features with more than one kingdom, and why the boundaries of Protista remain poorly defined even in the five-kingdom classification.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 8

Applied Concept

A geologist discovers a thick whitish sedimentary deposit at the bottom of an ancient lake bed. Chemical analysis reveals it is composed primarily of silica. With reference to this chapter, identify the origin of this deposit and describe its commercial uses.

The silica-rich sedimentary deposit is diatomaceous earth, formed by the accumulated cell walls of diatoms — microscopic chrysophytes belonging to Kingdom Protista that have silica-embedded, indestructible cell walls.
Over billions of years, diatoms that died settled to the bottom of water bodies, and their silica walls, being indestructible, accumulated as thick geological deposits — a process that has formed extensive diatomaceous earth strata worldwide.
Commercially, diatomaceous earth has valuable applications: its gritty, abrasive texture is used in metal polishing, and its porous structure makes it ideal for the filtration of oils, syrups, and other liquids in industrial food and beverage processing.
Diatoms are also the chief producers (primary producers) in ocean ecosystems, making their historical and ecological contribution immense — both as sediment-forming organisms and as the base of marine food chains.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 9

Applied Concept

A patient in a tropical country is hospitalised with recurring fever, chills, and anaemia. Blood smear analysis reveals an intracellular parasite in red blood cells. With reference to this chapter, identify the organism and the protozoan group it belongs to.

The organism is Plasmodium, the malarial parasite, belonging to the Sporozoans — a subgroup of Protozoans within Kingdom Protista.
Sporozoans are characterised by having an infectious spore-like stage in their life cycle. Plasmodium enters the human body through the bite of the female Anopheles mosquito and infects red blood cells.
The recurring fever and chills are caused by the synchronised rupture of infected red blood cells releasing merozoites — the characteristic cyclical pattern of malaria corresponds to the parasite's replication cycle within blood cells.
Malaria caused by Plasmodium has a staggering impact on human populations globally, making it one of the most significant infectious diseases, particularly in tropical and subtropical regions.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 10

Applied Concept

With reference to Ascomycetes, explain why Penicillium is considered one of the most medically important fungi. How was this medical importance discovered and what is the chemical basis of its action?

Penicillium is an ascomycete that produces penicillin, the first antibiotic discovered, which revolutionised the treatment of bacterial infections in the 20th century and saved millions of lives.
Penicillin was accidentally discovered by Alexander Fleming (1928) when he noticed that a Penicillium mould contaminant was killing Staphylococcus bacteria in a culture plate — the fungus was secreting a substance toxic to bacteria.
Penicillin acts by inhibiting the synthesis of the bacterial cell wall (peptidoglycan), causing the bacterial cell to lyse (burst) when it attempts to divide — this mechanism exploits the fundamental difference between fungal cell walls (chitin) and bacterial cell walls (peptidoglycan).
Penicillium exemplifies the economic importance of Kingdom Fungi as a source of antibiotics. Other ascomycetes like Aspergillus are also used industrially, illustrating how the fungal kingdom provides both beneficial compounds and useful enzymes for medicine, food, and industry.

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Chapter 2: Biological Classification | Application Questions

A microbiologist discovers an organism living in a highly acidic hot spring at 80°C. The organism has a unique cell wall not made of peptidoglycan. With reference to the Five Kingdom Classification, identify the group this organism belongs to and justify your answer.

During a water quality assessment, scientists find no lichen growth in an urban industrial area but abundant lichen growth in a rural forest nearby. With reference to the nature of lichens, explain what this observation indicates and why lichens are useful for such assessments.

A patient presents with neurological symptoms and is diagnosed with a fatal degenerative brain disease. Laboratory tests confirm the infectious agent contains no nucleic acid. With reference to the chapter, identify the agent and explain why it is challenging to classify and treat.

Bread left at room temperature in a warm, humid kitchen develops a black fuzzy growth within two days. With reference to Kingdom Fungi, explain the type of organism causing this, its mode of nutrition, and the conditions that favour its growth.

An aquatic ecologist observes that a small lake has suddenly turned red and fish mortality has increased sharply. With reference to the organisms discussed in this chapter, identify the cause and explain the biological mechanism behind this event.

Farmers in a village noticed that the soil in their paddy fields became naturally fertile over time without any chemical fertiliser addition, particularly in areas where blue-green algae were growing. With reference to this chapter, explain the scientific basis for this observation.

A geologist discovers a thick whitish sedimentary deposit at the bottom of an ancient lake bed. Chemical analysis reveals it is composed primarily of silica. With reference to this chapter, identify the origin of this deposit and describe its commercial uses.

A patient in a tropical country is hospitalised with recurring fever, chills, and anaemia. Blood smear analysis reveals an intracellular parasite in red blood cells. With reference to this chapter, identify the organism and the protozoan group it belongs to.

With reference to Ascomycetes, explain why Penicillium is considered one of the most medically important fungi. How was this medical importance discovered and what is the chemical basis of its action?

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